(a) Field of the Invention
The present invention relates to a method for modification of a microchip or a lab-on-a-chip. Specifically, the present invention relates to a method for modification of microchannels of a polydimethylsiloxane (PDMS) microchip.
(b) Description of the Related Art
Recently, development of microprocessors that are capable of integrating and conducting a series of processes required for analysis including injection of an analysis sample, pre-treatment, chemical reaction, detection, etc. in a miniaturized system, based on microfluidics, has been actively progressed. The system is referred to as a lab-on-a-chip or simply as a microchip. Currently, silicon, glass, quartz, etc., having electrical insulating properties and that are capable of transferring a solution under the electric fields, are widely used materials of microchips. The surface of glass has hydrophilicity, and thus channels of a glass microchip can be easily filled with an aqueous solution using the capillary phenomenon. And, it does not show surface swelling because it is chemically stable for most of solutions or gases. Further, glass can prevent small molecules in a solution from penetrating through the surface of the channels because it does not have small pores. Especially, its surface silanol group (—SiOH) imparts rapid electro-osmotic flow (EOF) and enables rapid transfer of a solution, and thus the glass microchip enables rapid analysis when conducting electrophoresis. A glass microchip is generally manufactured using photolithography. However, photolithography has defects in that, in general, it takes a long time to conduct the process and its manufacture cost is high. And, it is dangerous because microchannels are generally formed using caustics. Further, a process of joining upper and lower plates to form channels in a microchip is complicated and time consuming, and the plates are easily broken, and therefore it is difficult to widely apply photolithography for manufacture of a microchip.
Due to these limitations, a method for manufacturing a microchip rapidly and at an inexpensive cost using polymers is being actively studied.
Using soft lithography, a PDMS microchip can be manufactured by replica molding. According to the method, precursors are poured into a prepared mold and hardened at a low temperature. The method is simple and easy, and the manufacture cost is low thus enabling mass production. And, a thus-prepared PDMS microchip has good optical permeability even to UV, thus allowing detection of an analysis material by an optical method. Further, the prepared PDMS is chemically stable and does not react with other materials, it is nontoxic and thus suitable for living material, and therefore the importance thereof is increasing.
However, since the PDMS microchip has strong hydrophobicity due to repeated —Si—(CH3) at the surface thereof, it is difficult to fill it with an aqueous solution. It also has low and unstable EOF, and thus it is difficult to electrically transfer a solution and separate and detect a sample. Further, the structures of microchannels are easily deformed because the surface swells when an analysis sample such as a protein or a hydrophobic material is absorbed or a non-polar solvent is absorbed at its surface. For the above reasons, a PDMS microchip is difficult to apply for an organic synthesis reaction using an organic solvent that has frequently been attempted lately, as well as to a reaction using an aqueous buffer solution.
Therefore, currently, the results of many studies for facilitating electrophoresis using a PDMS microchip through the modification thereof and increasing the efficiencies of reaction and separation/analysis using a microchip by decreasing absorption of living materials thus decreasing loss of sample are being presented.
As the first example, excessive energy such as oxygen plasma, UV rays, or corona discharge is injected to silica to oxidize the surface thereof, thus obtaining a hydrophilic surface (Efimenko, K. Wallace, W. E.; Genzer, J. Journal of Colloid and Interface Science 2002, 254, 306). Although this method can result in high EOF, it has defects in that hydrophobic PDMS having a low molecular weight is diffused at the surface and thus hydrophilicity is maintained for a very short time.
As the second example, Ocvirk et al. disclosed a method for increasing quantity of electric charge at the surface by injecting a charged surfactant at critical micelle concentrations (CMCs) or less, thus making the hydrophobic tail thereof be absorbed at the surface of PDMS and the hydrophilic head come out of the surface (Ocvirk, G.; Munroe, M.; Tang, T.; Oleschuk, R.; Westra, K.; Harrison, D. J. Electrophoresis 2000, 21, 107).
According to the above two methods, a microchip is modified by injecting a solution including a compound that is active to the surface in the channels of microchip, and washing the channels with the solution, or flowing the solution together with an electrolyte. However, since these methods use electrostatic interactions, absorption and desorption reversibly occur at the surface, and thus coating stability is not good.
Accordingly, in order to solve the problem in terms of stability, many studies regarding permanent modification methods wherein a generally high energy is applied to the surface to increase surface activity and then covalent bonds are formed by chemical reactions are progressing.
Papra et al. treated a PDMS surface with oxygen plasma to oxidize the PDMS surface, and then reacted 2-[methoxy(polyethyleneoxy)propyl]trimethoxysilane and poly(ethylene glycol)di(triethoxy)silane (Si-PEG-Si) to form a covalent bond, thereby obtaining a surface that is capable of effectively preventing absorption of hydrophilic protein (Papra, A.; Bernard, A.; Juncker, D.; Larsen, N. B.; Michel, B.; Delamarche, E. Langmuir 2001, 17, 4090). Although this method is good in terms of coating stability compared to the above two methods, it is complicated and accordingly difficult to easily apply.